Helppp and explain too

Suppose that the price p (in dollars) and the weekly sales x (in thousands of units) of a certain commodity satisfy the demand equation 4p cubedplusx squaredequals38 comma 400. determine the rate at which sales are changing at a time when xequals80, pequals20, and the price is falling at the rate of $.20 per week.

Ineed me i stuck and i don’t want to get it wrong

Define multiplicative inverse. give an example of a number and its multiplicative inverse.